Polarimetry

Imaging polarimetry for Michelle was commissioned in December 2005/January 2006. This observation mode has been offered for science usage starting in semester 2006B. This page gives some guidelines on carrying out Michelle polarimetry observations. These guidelines may change as we get more experience with such observations. Spectral polarimetry is not yet commissioned and so is not offered at this time.

Basic characteristics of Michelle's imaging polarimetry mode

The Michelle polarimetry observations are made in single-beam polarimetry mode. The following estimates have been derived from the commissioning data and from the expected properties of the wave-plate.

The transmission of the N-band wave-plate is about 0.3; the transmission of the Q-band wave-plate is slightly lower, about 0.25

The N-band wave-plate has maximum efficiency, assumed to be 1, at 10.5 microns; it decreases by about 10 percent at the ends of the N-band window

The Q-band wave-plate efficiency is near 1 for the Qa filter

The wire grid efficiency is high, close to 100 percent

The instrumental polarization signature is at most 0.2 to 0.3 percent

The instrument does not introduce any measurable rotation in the polarization vectors (i.e. ANGROT as defined in the POLPACK software is zero within the uncertainties of the measurements)

The transmission values given in the first item of the above list are used in the Michelle ITC and are conservative estimates.

The Observing Process

Imaging polarimetry is carried out similarly to regular imaging, in chop-nod mode. The difference for polarimetric observations is that a half-wave plate is introduced into the optical path just after the light enters the instrument. The wave-plate is rotated to four positions (wave-plate angles of 0 degrees, 22.5 degrees, 45 degrees, and 67.5 degrees), which modulates any polarized radiation from the target object.

The observations are taken in such a way that two cycles of the wave-plate positions are observed for each nod position. The wave-plate is cycled through the positions in the following order: 0, 45, 45, 0, 22.5, 67.5, 67.5, 22.5 degrees. This order of observation has the same effect as ABBA nodding--it tends to cancel out linear drifts in the sky emission with time or position. The differences between the 0 degree and 45 degree positions and the 22.5 degree and 67.5 degree positions select two orthogonal polarization directions. Only linear polarization can be measured by Michelle.

To keep the total time per nod position short, only about 5 to 6 seconds is spent at each wave-plate position. The time required to move the waveplate introduces and extra overhead, so polarimetry is less efficient than direct imaging (see the observing overheads page for more details). The observations are taken with regular chopping, the amplitude and direction of which are chosen by the user in the OT as with regular imaging observations. As usual, the maximum chop amplitude is 15 arc-seconds so objects that are larger than this in both dimensions as seen on the sky cannot be properly imaged with Michelle.

Required Weather Conditions

Cloud Cover and Image Quality

The commissioning observations have shown that stable conditions are needed to get accurate polarimetry observations. Stability is required in both the seeing and the atmospheric transparency in the filter that is being used. Since poor seeing in the mid-infrared is usually associated with variability of the seeing, it makes no sense
to carry out polarimetry observations in seeing worse than IQ=70%. The atmospheric transparency in the mid-infrared is much more difficult to judge, or to relate to the cloud cover. However it seems likely that CC=50% (i.e. clear conditions) is going to be required to obtain good quality polarimetry data. Therefore PIs should request observations in conditions of CC=50% and IQ=70% or better, and are discouraged from requesting observations in conditions worse than this.

Water Vapour

The water vapour conditions required are strongly dependent on the filter that is being used; see this Mid-IR Resources page for more details. From these considerations it is suggested that PIs request better water vapour conditions for the filters that are most sensitive to it -- Si-1, Qa, and Si-6. They need WV=50% conditions. The other filters can be used in poorer WV conditions, and the N' and Si-5 filters can be used under most WV conditions that are seen at Mauna Kea. Then the WV is very high it is likely the CC conditions will be too poor for polarimetry anyway, so WV=Any can be requested for the N' and Si-5 filters. Once we have more experience with imaging polarimetry more specific WV ranges that can be used for observations in different filters may be given.

Sky Brightness

As for most observations in the mid-infrared, the sky brightness (which refers to the lunar phase) should be set to SB=Any.

Airmass Considerations

It is probably a bad idea to attempt imaging polarimetry on targets that are at high airmass, because conditions are less stable at low elevations in most circumstances.

S/N Considerations

As well as needing stable conditions, polarimetry observations require good signal-to-noise ratio for accurate results to be obtained. The requirement is that one needs S/N of 70.5 in the reduced Stokes I image (i.e. in the total intensity image obtained by combining the images from the four wave-plate positions) to have an accuracy of +/- 1% in measured polarization. Obtaining this accuracy is made more difficult because the half-wave plates have a through-put of about 0.3, so for a given total time on-source a polarimetry observation will have a S/N value about 3 times lower than that which would be obtained in regular imaging mode with the same filter and the same total time on-source. Therefore to get the same S/N in polarimetry mode as in regular imaging mode takes of order ten times longer.